A battery is two or more electrochemical cells connected in series that store chemical energy and make it available as electrical energy. Common usage has evolved to include a single electrical cell in the definition. There are many types of electrochemical cells, including galvanic cells, electrolytic cells, fuel cells, flow cells, and voltaic piles. A battery's characteristics may vary due to many factors including internal chemistry, current drain, age and temperature.
Batteries can be employed in a wide range of electronic circuits. Some batteries can be recharged. There is a known relationship between a battery's capacity, e.g., its ability to deliver power to a load, and its internal resistance or impedance. Thus, battery impedance monitors can be employed to determine a remaining battery capacity.